A very common type of lock in widespread use today consists of a stationary outer cylinder and a rotatable plug. The plug has a number of transverse passages, each containing a tumbler. In some cases each tumbler is kept in its inoperable position by a spring and the key acts against the spring so as to position the tumbler into the desired position so that the lock can be opened.
In other locks, the tumblers are free to be positioned by a double bitted key. Such tumblers do not use springs. It is in relation to such locks that this invention is made.